As the image forming method using a static latent image developing method, typically an electrophotography, those described in U.S. Pat. Nos. 2,297,691 and 2,357,809 are cited. In such the methods, an image is finally formed by a process for forming a static latent image on a photoreceptor, a developing process for developing the static latent image to a toner image by a dry type developer containing at least a toner for developing the static latent image, a transfer process for transferring the toner image onto an image recording material such as paper, and a fixing process for fixing the toner image on the image recording material by heat or pressure.
In such the method, it is necessary for forming the final image with a suitable quality for a long period that the toner has high fluidity and stable electrification property. For improving the fluidity of the toner, a method by the addition of a fluidity improving agent such as silica fine powder as an external additive has been known.
As above-mentioned, the toner image is formed by developing the static image in the developing process. However, the toner forming the toner image is not completely transferred onto the recording material, also referred as to transferring material or image supporting material, and a part of the toner is usually remained on the photoreceptor.
The remained toner is usually recovered by a cleaning device and discarded. Recently, an image forming apparatus is noted from the economical or environmental viewpoint, which has a toner recycle system in which the retrieved toner is restored by a toner conveying screw to the developing device for reusing the developing toner or a cleaning-less system in which the residual toner is restored by the developing device on the occasion of the next image forming action.
However, the toner is received excessive physical compressing force by a toner recovering device such as the toner conveying screw when the usual small diameter toner containing the fluidity improving agent is applied to the image forming apparatus using the toner recycle system or the cleaning-less system. Consequently, the fluidity improving agent or the external additive to be on the surface of colored resin particle is buried into the colored resin particle so that the fluidity of the toner is gradually lowered and the electricity is changed, and problems such as scattering of toner, occurring of fog and lowering of image density are occurred.
Techniques for inhibiting the degradation of the fluidity of the toner caused by burying of the fluidity improving agent into the colored resin particle are proposed in, for example, Japanese Unexamined Patent Application Publication, hereinafter referred to as JP-A Nos. 9-211883, 10-104869 and 10-198063. JP-A Nos. 5-224456 and 2000-298372 describe technology in which a combination of an external additive with a large particle diameter and that with a small particle diameter is applied to the toner.
However, any method cannot sufficiently solve the problems at the present time. Recently, a toner with small diameter, particularly a polymerized toner, is noted for raising the image quality accompanied with the digitalization of the image forming apparatus. Moreover, the improvement of the transfer property for transferring the toner image with high fidelity without deformation is come as the theme of the investigation in the field.
It has been known that the toner produced by the polymerization method has merits that the particle diameter distribution and the charge distribution of the colored particles are sharp and the toner is excellent in the transfer ability. The method for forming the image without use of the cleaning device is going to be practically used. However, the reliance of the method is insufficient since it is difficult to completely remove the toner from the photoreceptor under any conditions of temperature and humidity.
A method by adding an external additive with a large particle diameter has been known for improving the transfer ability of the toner, for example, that disclosed in JP-A No. 11-38668. There is a problem on the external additive with the large that such the external additive is difficultly fixed with the colored particle so that the external additive released from the colored particle is transferred and accumulated on the photoreceptor even though such the external additive shows an effect to reduce the adhesive force of the toner with the photoreceptor.
Particularly in low temperature and low humidity condition, the released external additive is strongly charged and accumulation on the photoreceptor which finally causes a damage of the photoreceptor and a fault of the image.